WASHINGTON - The United States, South Korea and Japan are sniffing the air -- literally -- in a coordinated intelligence effort to determine what type of nuclear device was detonated by North Korea's secretive regime.

The country's two previous underground nuclear tests, in 2006 and 2009, drew on its limited reserves of plutonium.

Any evidence that North Korea used highly enriched uranium this time would signal that it has developed a second source of fissile material, expanding its potential warhead capabilities and raising the risk that the cash-strapped nation might sell such uranium to would-be nuclear weapons states such as Iran.

Air-sampling equipment, some in aircraft and some at ground-based facilities, is sniffing for residual radioactive emissions from the underground tests that would confirm it was a nuclear explosion and provide markers for the type of fissile material used.

A similar inquiry was successful in 2006, while one in 2009 failed because that test didn't vent radioactive material.

"One important question is whether the nuclear test used only plutonium or involved highly enriched uranium, either alone or in combination with plutonium," said David Albright and Andrea Stricker of the Institute for Science and International Security, a Washington-based research group that tracks nuclear issues.

North Korea tested "a smaller and light" nuclear device on Tuesday, the official Korean Central News Agency said, two months after test-firing a long-range rocket. The device had a yield of 6 to 7 kilotons, South Korea's Defense Ministry estimated.

That was bigger than the previous two North Korean detonations, though less than half the explosive power of the uranium-fueled bomb that the United States dropped on Hiroshima, Japan, in 1945.

Much of the policy debate since Tuesday's blast has focused on indications that North Korea was testing a miniaturized nuclear device as the regime seeks to develop a warhead that could be placed on a long-range missile.

The U.S. intelligence community assesses that North Korea remains some years from achieving the capability to threaten the continental United States, though it might pose a nuclear threat to its regional neighbors sooner.

North Korea said in its statement that the nation now has achieved a "diversified" nuclear capability -- a comment that drew attention as a possible signal that uranium was used.

U.S. Secretary of State John Kerry said that North Korea's nuclear test poses a threat to global peace.

"The international community needs to come together for a swift, clear response," Kerry said. "This is about proliferation, and it's also about Iran because they're linked."